Heat small sauté pan over medium heat. Add 1 tbsp. olive oil. When oil is hot, add onions and sauté until soft and golden, about 5 to 7 minutes. Set aside.

To assemble the casserole: Add a layer of potato slices to the baking dish, overlapping them slightly. Then sprinkle with some onions, tomato slices, garlic, olives, capers and red peppers. Season with herbes de Provence, and salt and pepper. Sprinkle half the salt cod over top. Drizzle with olive oil. Repeat layers, finishing with the salt cod. Drizzle with more olive oil.

Cover and bake, until potatoes are fork tender, about 1 hour 30 minutes.